The invention relates to well logging for evaluating the resistivity of earth formations through which a borehole passes, and more particularly to an electrical logging method and apparatus using electrodes.
A well logging device with electrodes that has been commercially available for many years and that is known under the name "Dual Laterolog" is described in U.S. Pat. No. 3,772,589 (Scholberg). That device comprises an array of annular electrodes used for sending electrical measurement currents into the formations for the purpose of measuring their resistivity. The measurement currents are focused in an annular zone having the form of a disk perpendicular to the borehole axis by means of auxiliary currents emitted by guard electrodes. That device includes measuring deep resistivity of the earth formations (LLd mode) and measuring shallower resistivity of the earth formations (LLs mode) by emitting currents at different frequencies, typically 35 Hz and 280 Hz.
A drawback of that device is that its longitudinal resolution (i.e. in the longitudinal direction of the borehole) is poor, being about one meter. In addition, in some cases and in particular in boreholes that are deviated or horizontal, the annular zone scanned by the device around the borehole comprises different layers of earth formation and gives an average measurement that has little meaning. It is therefore desirable to obtain resistivity measurements in a plurality of azimuthal directions around the borehole.
Logging apparatuses are known that detect the resistivity of formations in a plurality of azimuthal directions around the axis of the borehole. Those devices seek to obtain information about fractures or about the dip of formations. For example, British patent 928 583 (British Petroleum Company Limited) describes an array of azimuthal measurement electrodes distributed circumferentially around the periphery of a logging sonde. A guard electrode which surrounds the measurement electrodes enables an auxiliary current to be emitted for focusing the currents emitted by each of the measurement electrodes.
A difficulty with those apparatuses is obtaining effective focusing of the azimuthal measurement currents.
The radial investigation depth obtained with azimuthal electrodes is generally less than that obtained with a deep Laterolog device of known type. It is therefore desirable to provide conventional deep Laterolog measurements in addition to azimuthal resistivity measurements.